PPARa regulates ER-lipid droplet protein Calsyntenin-3b to promote ketogenesis in hepatocytes

Ketogenesis requires fatty acid flux from intrahepatic (lipid droplets) and extrahepatic (adipose tissue) lipid stores to hepatocyte mitochondria. However, whether inter-organelle contact sites regulate this process is unknown. Recent studies have revealed a role for Calsyntenin-3b (CLSTN3b), an endoplasmic reticulum-lipid droplet contact site protein, in the control of lipid utilization in adipose tissue. Here, we show that Clstn3b expression is induced in the liver by the nuclear receptor PPARa in settings of high lipid utilization, including fasting and ketogenic diet feeding. Hepatocyte-specific loss of CLSTN3b in mice impairs ketogenesis independent of changes in PPARa activation. Conversely, hepatic overexpression of CLSTN3b promotes ketogenesis in mice. Mechanistically, CLSTN3b affects LD-mitochondria crosstalk, as evidenced by changes in fatty acid oxidation, lipolysis-dependent mitochondrial respiration, and the mitochondrial integrated stress response. These findings define a function for CLSTN3b-dependent membrane contacts in hepatic lipid utilization and ketogenesis.